The Surface Acoustic Wave (SAW) is used as the basis for a variety of electronic devices including delay lines, filters, and correlators which are key components in numerous electronic applications. Radio frequency (RF) SAW filters have a number of desirable characteristics with respect to their size and cost along with their electronic properties such as insertion loss, bandwidth, transfer function, etc. Numerous advances in this field have led to greatly improved SAW device reliability and performance, and RF SAW filters have found widespread application in modern mobile communications equipment such as cellular telephones. However, each new development in mobile communications has required a new filter design matched to the new frequencies and spectral characteristics of the transmitted waveforms. As an alternative to regular re-design and development of new hardware, it is desirable to have a tunable RF SAW filter capable of providing a variety of specified electrical characteristics, such as low insertion loss, tunable bandwidth, tunable frequency, and tunable filter transfer function.
Prior developments in the field of programmable transversal filters based on SAW devices and closely related fields that provide useful guidance on these choices are reviewed in “Design of a Selectable Performance Front End Filter Using Acoustic Surface Wave Resonators,” by R. Pastore, J. A. Kosinski, W. N. Porter, and H. L. Cui, in Proceedings of the 1997 IEEE International Frequency Control Symposium, May 1997, pp. 858-866. Transversal filters utilizing a SAW device and programmable transversal filters have found many different applications. Examples of transversal filters include Zimmerman et al. U.S. Pat. No. 4,752,750 “Hybrid Programmable Transversal Filter,” issued on Jun. 21, 1988, Birkett et al. U.S. Pat. No. 5,051,709 “SAW Device Tapped Delay Line and Equalizer,” issued on Sep. 24, 1991, Smythe, Jr. et al. U.S. Pat. No. 5,194,837 “Multi-Tap Programming Circuit for Transversal Filters,” issued on Mar. 16, 1993 and Zimmerman et al. U.S. Pat. No. 5,387,887 “Miniature Digitally Controlled Programmable Transverse Filter Using LSI GAAS Integrated Circuits,” issued on Feb. 7, 1995. These patents disclose programmable transversal filters that obtain some of the desired electronic characteristics but still suffer from a number of deficiencies such as triple transient echo, conversion loss and an insertion loss penalty associated with both imperfect energy conversion and propagation loss.
The improved filters disclosed in Kosinski et al. U.S. Pat. No. 6,459,345 “Programmable SAW Filter Including Unidirectional Transducers,” which disclosed incorporating a finger array, phase shifters and gain devices to eliminate bi-directional energy loss, and Kosinski et al. U.S. Pat. No. 7,132,908 “Selectable Performance Interference Filter” which disclosed the use of asynchronous IDT finger arrays, have reduced and minimized the deleterious effects of triple transit echo and the insertion loss penalty to a great degree while enabling a greater flexibility than previously possible. However, the complete elimination of the negative effects of the insertion loss penalty is not possible with any of the prior art arrangements. The energy conversion losses at the SAW input and output IDT arrays and the SAW propagation loss all contribute to undesirable device insertion loss. While the use of unidirectional transducers as in Kosinski et al. U.S. Pat. No. 6,459,345 helps to minimize the conversion loss, the propagation loss is inherent in the device and cannot be eliminated. The propagation loss can be compensated to some level through additional gain in the tap weighting circuitry, however, the gain cannot be increased arbitrarily while maintaining a desired weighting function, and hence the overall filter will still suffer from a certain amount of loss and an attendant noise figure.
In addition to some residual insertion loss, the improved filters disclosed in Kosinski et al. U.S. Pat. No. 6,459,345 and No. 7,132,908 also encumber the unforeseen difficulty of strong signal capture. While the incorporation of a tunable input IDT provides clear advantage in enabling greater flexibility with respect to the filter transfer function, the tunable input IDT necessarily places gain elements ahead of the filter transfer function where they are susceptible to being saturated by strong signals outside of the intended filter bandwidth. Such is not a significant drawback in intermediate filter (IF) applications where out-of-band signal levels are somewhat limited. However, in pre-selector applications where one expects large interference signals, the prior art programmable SAW filters of U.S. Pat. No. 6,459,345 and No. 7,132,908 are susceptible to deleterious signal capture effects that pre-selector filters are otherwise meant to overcome and obviate.
The common flaw with prior art programmable SAW filters, especially with respect to pre-selector applications, is that they are inherently “in-line” devices requiring the RF signal to pass through all of the RF filter component parts, with the exception of the control circuitry. The “in-line” configuration inherently places broadband gain elements ahead of any SAW-based filter transfer function, and thus is inherently and unavoidably subject to strong signal capture effects. Thus, there has been a long-felt need for programmable SAW filters and filter components which are not “in-line” devices and do not suffer from the undesirable drawbacks, limitations and shortcomings associated with insertion loss, propagation loss, and strong signal capture effects.